1. Field of the Invention
This invention is related to the field of electrical measurements, more particularly, to current sensing.
2. Description of the Related Art
Measuring, sensing and/or controlling current flow into and out of electronic circuits is an important requirement in many electronic systems, spanning a wide range of applications. There are numerous different current sensing techniques, which are as diverse as the applications that require monitoring current flow. Examples of systems that may rely on current sensing to perform essential functions include programmable current sources, overcurrent-protection and monitoring devices, various low-current systems, battery chargers, various power supplies, and circuits where the ratio of output current to input current is of importance. In addition, portable applications may also require dedicated current monitors that fit in a small package and operate with low quiescent current.
The two most common current-measurement/current-sensing methods are low-side current sensing and high-side current sensing. In the case of low-side current sensing, the current is sensed in the ground return path of the power connection to the monitored load or device under test (DUT), and generally flows in just one direction. Any switching that is performed is on the load-side of the current sense circuit. In the case of high-side current sensing, the current is sensed in the supply path of the power connection to the monitored load (in other words, the sensing path in this case typically includes the supply terminal of the load). Current generally flows in just one direction, with switching performed on the load side of the current sense circuit.
The low-side and high-side configurations have trade-offs in different areas. For example, the low-side resistor adds undesirable extraneous resistance in the ground path, while circuitry associated with the high-side resistor has to withstand relatively large common-mode signals. Overall, advantages of low-side current sensing include low input common-mode voltage, ground referenced output voltage, and easy single supply design. Disadvantages of low-side current sensing include the load being lifted from direct ground connection. Overall, advantages of high-side current sensing include the load being grounded. Disadvantages of high-side current sensing include high input common-mode voltages, and the need to level shift the output down to system operating voltage levels.
Full-range (i.e. high and low side) current sensing configurations are also used, where a bidirectional current is sensed in a bridge driven load, or unidirectional high side connection with a supply side switch. While full-range current sensors may require only one current sense resistor for bidirectional sensing, and feature convenient sensing of load current on/off profiles for inductive loads, they're also prone to wide input common-mode voltage swings, with the common-mode rejection also limiting high frequency accuracy in PWM applications. Some applications may require current sensing over a wide range of current values, with increased resolution when measuring smaller currents. Many present solutions for sensing current over a wide dynamic range use two current sense amplifiers with two values of sense resistors and a comparator, to allow for higher sensitivity and resolution of measurement for low currents while also sensing higher currents.